1. Field of the Invention
The invention relates to a method of manufacturing a bending-resistant, torsionally yielding tubular profiled member as a transverse support for a twist beam rear axle of a passenger car.
2. Description of the Related Art
Various suggestions are known in regard to designs of transverse supports of a twist beam rear axle of a passenger car in order to provide them, on the one hand, with bending resistance and, on the other hand, with a sufficient torsional yield. Prior art references concerned with these problems are, for example, EP 0 249 537 B1, EP 0 681 932 A2, EP 0 752 332 B1, DE-Gm 297 20 207, and U.S. Pat. No. 2,069,911.
All of these transverse supports have in common that their manufacture overall is complex, and, accordingly, the production costs within the context of a complete twist beam rear axle are high.
It is an object of the present invention to provide a method of manufacturing a bending-resistant, torsional yielding tubular profiled member as a transverse support for a twist beam rear axle for a passenger car which not only ensures an increased economical production but also, with respect to the respectively required conditions, makes it possible to design the individual process parameters variable as much as possible.
In accordance with the present invention, this is achieved according to a first variant of the method in that a tubular blank of tempering steel is first provided by U-shaped cold forming with a torsionally yielding central longitudinal section while ensuring torsion-proof end sections, whereupon the thus formed tubular profiled member is annealed at least in partial sections thereof at a temperature level between 850xc2x0 C. and 960xc2x0 C., is subsequently hardened in water at a temperature above the AC3 point, is subsequently tempered at a temperature between 200xc2x0 C. and 550xc2x0 C. for a duration of more than 5 minutes, is subsequently subjected to at least an outer surface hardening, and is finally subjected to further configuration processing steps for completing a twist beam rear axle.
Accordingly, a tube blank, independent of its cross-section, of tempering steel is being used for the manufacture of a tubular profiled member as a transverse support for a twist beam rear axle. First, the tube blank is provided by means of U-shaped cold forming with a central longitudinal section that is torsionally yielding while ensuring at the same time torsion-proof end section. The configuration of the end sections can be as desired. It can be circular or shoe-shaped, optionally with incorporation of stay-shaped sheet metal. Also, reinforcement embossments can be provided in the transitional sections between the U-shaped central longitudinal section and the end sections.
After cold forming, the now present tubular profiled member is subjected to a heat treatment in which, at least in partial sections thereof, it is annealed at a temperature level between 850xc2x0 C. and 960xc2x0 C. In this context, the term partial sections refers preferably to the end sections of the tubular profiled member. Subsequently, the tubular profiled member is hardened in water at a temperature above the AC3 point of the iron-carbon-diagram and is subsequently tempered at a temperature between 200xc2x0 C. and 550xc2x0 C. for a duration of more than 5 minutes.
A tubular profiled member formed and heat-treated such exhibits then a high permanent strength under dynamic load, as is desirable especially in regard to the aforementioned torsional profiled member as a component of the twist beam rear axle of a passenger vehicle.
Subsequent to the heat treatment, the tubular profiled member is subjected to at least an outer surface hardening. This refers primarily to a mechanical surface hardening. Optionally, it is also possible to perform an additional inner surface hardening, especially by mechanical means.
Finally, the thus manufactured tubular profiled member is subjected to further configuration processing steps for completion of a twist beam rear axle.
An especially advantageous embodiment of the afore described variant is provided when the tubular profiled member is annealed at a temperature level between 920xc2x0 C. and 950xc2x0 C., especially approximately 930xc2x0 C.
In this context it is especially important when the annealed tubular profile is then tempered at a temperature of approximately 280xc2x0 C. for a duration of approximately 20 minutes.
In the context of the invention it is also advantageous when a tube blank of the material specification 22MnB5 is used for the manufacture of the tubular profiled member as a transverse support of a twist beam rear axle.
In accordance with the present invention, the object of the invention is moreover achieved according to a second variant of the method in that a tubular blank of case-hardening steel is first provided by U-shaped cold forming with a torsionally yielding central longitudinal section while ensuring torsion-proof end sections, whereupon the thus formed tubular profiled member is subjected to case hardening at least in partial sections thereof during the course of a heat treatment by carburization of the surface layer of the tubular profiled member with subsequent quenching, is subsequently subjected to at least an outer surface hardening process, and is finally subjected to further configuration processing steps for completion of a twist beam rear axle.
In the context of this variant a case-hardening steel is used. In this variant, a tube blank of a case-hardening steel is also first provided by U-shaped cold forming with a torsionally yielding central longitudinal section. The end sections of the tubular profiled member are, however, torsion-proof. As in the first variant, the end sections can be of different configurations.
Subsequent to the cold forming step, the thus formed tubular profiled member is case-hardened at least in partial sections thereof during the course of a heat treatment by carburization of the surface layer of the tubular profiled member with subsequent quenching. According to the invention, this also ensures that a cold-formed tubular profiled member as a transverse support in the context of a finished twist beam rear axle is provided with a high permanent strength under dynamic load.
After the heat treatment process an outer surface hardening is carried out. Subsequently, the tubular profiled member is subjected to further configuration processing steps for completion of a twist beam rear axle.
A preferred steel specification (quality) for forming a bending-resistant, torsionally yielding tubular profiled member is C15.
For the first as well as for the second variant of the method according to the invention, it is very advantageous when the heat treatment of the cold-formed tubular profiled member is performed in the transition sections between the U-shape central longitudinal section and the torsion-proof end portions.
Moreover, it is advantageous when the surface hardening of the profiled member is carried out by bombarding with balls, especially steel balls.